With the opening of the No. 7 line extension just months away, Mayor de Blasio’s refusal to spend an extra dime of city funds to bolster the MTA’s capital budget betrays the hollowness of his “progressive” agenda.

Yes, the opening of the new line from Times Square to 11th Avenue and West 34th Street has been postponed again, from April as previously announced to July. It follows earlier delays, and there might be more.

But the real news is not another, irksome minor setback, but that the No. 7 opening remains imminent by any time frame that matters. Even if it takes a few months more — let’s hope not — it will mark a signal moment in the city’s history.

Yet, predictable “here-we-go-again” press coverage threatens to overwhelm what counts: the permanent, easy linkage of the Far West Side with Midtown, which makes all the former backwater-district’s blossoming possible.

It distracts as well from the gulf between former Mayor Michael Bloomberg’s contribution to mass transit and Mayor de Blasio’s “let Albany and the feds pay for everything” stance.

Mega-billionaire Bloomberg is entirely to thank for bringing the No. 7 far west — mostly paid for not by the MTA but with $2 billion in city bonds issued in 2006.

To pay off the bonds, Bloomberg counted on future real-estate tax revenue from construction projects he correctly reckoned would rise in anticipation of the new station.

Yet “progressive” de Blasio, who surely knows how indispensable reliable mass transit is to lower-income New Yorkers, won’t lift a finger to help the MTA out of a looming capital budget crisis.

(The MTA is in the hole for $15.2 billion, on a proposed $32 billion, five-year budget desperately needed for infrastructure fixes.)

De Blasio has balked at upping the city’s measly $100 million annual contribution to the MTA’s capital plan to a barely less measly $125 million — still far below the $363 million it would be if adjusted for inflation since 2000, according to the Independent Budget Office.

He instead wastes city funds on futile initiatives like $35.3 million to reduce violence in the jails. Such warped priorities make it impossible to conceive of him getting a real-world project like the No. 7 line out of the barn.

Make no mistake: It’s the city’s most important new mile of tracks in 100 years.

They traverse the 12 longest blocks on the map — as anyone who’s strolled the windswept route knows.

Without the lengthened No. 7, there would be no new office towers rising for Time Warner, Coach Inc., L’Oreal and Skadden Arps. Nor would planned new apartment buildings, hotels and public parks have gotten off the drawing boards.

It’s likely de Blasio doesn’t have a clue about any of this. He bizarrely boasts of having never been to the High Line Park.

Since the park’s north entrance is steps from the new station, it’s unlikely the mayor has much interest either in the once-desolate zone around it.

In fairness, de Blasio isn’t the only one to blame. While he at least pays lip service to the subways, self-described “car guy” Gov. Cuomo calls the MTA’s $32 billion capital plan “bloated.”

Yet, as Nicole Gelinas has written, much of the alleged waste is Cuomo’s fault, for forcing the agency to pledge hundreds of millions of dollars out of future revenues to its unions.

Sometimes, you need the bloat to get the bone. Bloomberg, who fought his own battles with the MTA, understood that.

Maybe, when the No. 7 makes its first run to 11th Avenue, de Blasio and Cuomo will finally get it, too.